1. Field of the Invention
The present invention relates to a pressurized oil supply/discharge circuit with a residual pressure holding function and a residual pressure holding valve device for use in the pressurized oil supply/discharge circuit, all of which are adapted to leave a predetermined pressure remaining within a hydraulic actuation chamber even in the event that a breakage of a hydrualic hose and the like causes an oil leak in a pressurized oil supply/discharge passage connected to the hydraulic actuation chamber of a hydraulic cylinder.
2. Description of the Prior Art
A known pressurized oil supply/discharge circuit with a residual pressure holding function has been disclosed in Japanese Patent Laid Open Publication No. 1979-108156.
As shown in a circuit diagram of FIG. 17, this publication discloses a hydraulic cylinder applied to a clamping device.
That is, a clamping hydraulic actuation chamber 203 is formed in the one side of a piston 202 of a double-acting type hydraulic cylinder 201, a check valve 205 with a forcibly valve-opening (valve actuating) function is disposed in a clamping pressure oil supply/discharge passage 204 connected to the hydraulic actuation chamber 203, and an unclamping pressure oil supply/discharge passage 207 is connected to an unclamping actuation chamber 206 formed in the other side of the piston 202.
At the time of clamping, actuating oil is discharged from the unclamping hydraulic actuation chamber 206 and the pressurized oil is supplied to the clamping hydraulic actuation chamber 203, so that the hydraulic cylinder 201 is operatively contracted to press and fix a fixed or clamped object 210 by means of a clamping member 209. Under this clamped condition, when a pressure on the inlet side of the check valve 205 abnormally drops due to a breakage of a hydraulic hose and the like, the check valve 205 serves to prevent a counter flow so as to leave or maintain a pressure within the clamping hydraulic actuation chamber 203 and thus to prevent a shift and/or a fall of the fixed object 210. Incidentally, slight leakage of pressure from the check valve 205 is supplemented by means of an accumulator 211.
On one hand, at the time of unclamping, when the pressurized oil is supplied from the unclamping oil supply/discharge passage 207 to the unclamping hydraulic actuation chamber 206, the check valve 205 is opened by means of hydraulic pressure via a pilot oil passage 213, so that a discharge of oil from the clamping hydraulic actuation chamber 203 is allowed and the hydraulic cylinder 201 is operatively extended.
The above-mentioned prior art has the advantage that the clamping condition can be held by means of a one-way checking function of the check valve 205 even though a leakage of pressurized oil is caused on the inlet side of the check valve 205, but there is the following problem associated therewith.
For example, like in a hydraulic clamping device for a metal mould of an injection moulding machine, when the hydraulic cylinder is subjected to a raised temperature under the clamping condition, pressure within the clamping hydraulic actuation chamber 203 can gradually increase or build up due to volumetric expansion of the trapped pressure oil. Accordingly, the clamping force of the hydraulic cylinder 201 increases sufficiently great so as to cause concern over damage of a clamped or fixed object 210.